The invention relates to adaptive equalizers for digital communications and is especially, but not exclusively, applicable to adaptive equalizers of the decision feedback type for reducing intersymbol interference in subscriber loops of telephone systems.
During transmission, digital signals are attenuated and distorted by the filter characteristics of the transmission path. Usually such attenuation and distortion are equalized by a so-called .sqroot.F "equalizer". However, such an equalizer cannot compensate for reflections or echoes which are caused by discontinuities, such as wire gauge changes or bridged taps, in the transmission path. These echoes or reflections arrive at the end of the path later than the pulse which initiated them--possibly several bit periods later. As a result, for example, a digital zero following a digital "one" will not be truly zero because of the presence in its bit period of energy from the preceding pulse, and possibly earlier pulses. This and the attenuation of the main pulse, may make it difficult for the usual threshold or decision circuit to decide accurately whether a particular symbol should be a 1 or a 0. This phenomenon of intersymbol interference, generally known as "closing of the eye", is much worse in the loops because of the common occurrence of bridged taps--open-ended lines or stubs connected to the loop between its ends.
To reduce intersymbol interference an additional equalizer is provided at each receiver. Typically the additional equalizer includes a transversal filter through which the received signal is passed. Various proportions of the filter outputs are chosen to map the interference and are fed back as a correction signal for subtraction from the incoming signal.
The amplitude and timing of the echoes or reflections vary with each different transmission path or loop and it is preferable for the equalizer to be adaptable to compensate for the variations. This has led to the development of adaptive equalizers in which the coefficients which determine the feedback or correction signal are continuously adapted and optimized during transmission.
In aaking such adaptive equalizers for digital signals, problems are encountered due to "hunting" or fluctuations in the absence of intersymbol interference, limited precision due to D/A quantization, or random variation of coefficient values due to additive noise. Also they are not particularly immune to fixed offset and misalignment effects. Solution of these problems tends to increase complexity and cost.
As digital telephony has now developed to the stage that digital transmission in the ordinary two-wire subscriber loops is desirable, so considerable numbers of equalizers are involved; there is a need for an adaptive equalizer which not only mitigates aforementioned problems but is relatively simple, cheap to make, and rugged. The present invention aims to satisfy this need.